Both the investment casting process and the lost wax shell mold building process are well known, for example, as is apparent from the Operhall U.S. Pat. Nos. 3,196,506 and 2,961,751. The lost wax shell-mold building process involves repeatedly dipping a wax or other fugitive pattern of the article to be cast in ceramic slurry to provide a ceramic slurry layer, draining excess slurry, stuccoing the slurry with coarse ceramic particles to provide a stucco layer on the slurry layer, and drying the layers to build up a shell mold of desired wall thickness on the pattern. The green shell mold/pattern assembly then is subjected to a pattern removal operation to selectively remove the pattern from the shell mold. A commonly used wax pattern removal technique involves flash dewaxing where the green shell mold/pattern assembly is placed in an oven at elevated temperature to rapidly melt the wax pattern from the green shell mold. Following pattern removal, the green shell mold is fired at elevated temperature to develop mold strength for casting of molten metal or alloy therein.
Conventional lost wax ceramic shell molds can be prone to mold cracking or splitting during the pattern removal operation described above.
Attempts have been made to raise the capability of ceramic shell molds in the DS casting of superalloy components. For example, US Reissue 34,702 describes in one illustrative embodiment wrapping alumina-based or mullite-based reinforcement fiber in a continuous spiral about an intermediate mold wall thickness as it is being built-up. U.S. Pat. No. 6,364,000 discloses in one illustrative embodiment positioning one or more continuous carbon-based reinforcement fibers in a ceramic shell mold wall to this end.
However, ceramic investment casting shell molds have much higher compressive strength than tensile strength. After a shell mold is cast with molten metallic material at high temperature, the shell mold is allowed to cool to room temperature. During the cooling period, the molten material in the shell mold will consolidate and solidify from liquid state to solid state to form a casting. Once the metallic material becomes solid, the thermal expansion coefficient of the metallic material is much higher than that of the ceramic shell mold. Therefore, the thermal shrinkage of the metallic material (the casting) is constrained by the ceramic shell mold. The dimensional mismatch between the solid casting and the shell mold results in a general tensile stress on the solid casting and compressive stress on the ceramic shell mold. This general stress on the solid casting is typically enhanced at certain areas thereof due to complex casting geometries and can result in serious casting defects, such as casting cracks (hot tears), grain recrystallization in single crystal castings, and other defects.